Coal fired steam generating furnaces have become more efficient by increasing the temperature of the steam produced in the super heat section of the boiler. Boilers, particularly tubing within the boiler, have been improved so as to no longer be a limiting factor in obtaining high temperatures. In coal fired steam generating furnaces the limiting factor in obtaining high temperatures in the super heat section of the steam boiler is the build-up of ash produced by the combustion of coal, often referred to as clinker, on the boiler tubes.
The fine ash produced by the combustion of the coal is normally carried with the combustion products through the maze of boiler tubes. However, if the boiler tubes are hot enough to exceed the fluxing temperature of the ash, the ash will adhere to the tubes much the same as snow being driven onto a warm masonry wall. The ash deposited on the boiler tubes has an insulating effect and can build-up to the point of interfering with the movement of combustion gases through the furnace.
Ash build-up on the tubes is usually detected by measuring and comparing the pressure of combustion gases as they pass through the banks of tubes. A reduction of pressure above the banks of tubes indicates a build up of ash or clinker on the boiler tubes. As the magnitude of the pressure drop increases, the rate of steam production must drop and the efficiency and the capacity of the boiler system is diminished.
Coal of the type producing a relatively low amount of ash upon combustion has been used to reduce the amount of ash deposited on the boiler tubes. However, the sodium content of low ash coal is often higher than the sodium content of high ash coals. Sodium lowers the melting temperature or softing temperatures of the uncombustible components making up the ash; the higher the sodium content the lower the softing temperature. Thus, a coal with an average ash content of nine percent may have a considerably higher sodium to ash ratio than a coal with an average ash content of seventeen percent ash. The lower ash coal may actually build up more ash deposit on the boiler tubes in a given time than the higher ash content coal.
Eventually, the boiler tubes must be cleaned to remove the ash build-up and return the boiler system to operating efficiency. Normally, high pressure steam jets are directed at the boiler tubes to blast away and break loose the deposits of ash. Deposits of ash that are resistant to the high pressure steam jets may be shot at with shotgun shells. Normally, shotgun shells are satisfactory to dislodge enough of the ash deposits to allow the boiler system to operate at normal efficiency; however, there are disadvantages to the use of shotgun shells to dislodge ash deposits. There are inherent dangers in firing a shotgun within the confines of a furnace at overhead metal structures. It requires skillful aim to dislodge a deposit of ash from boiler tubes. Shotgun shells are expensive and even with good aim a large number of shells may be required. Firing from a close range, to improve the efficiency of the operation, may damage or puncture boiler tubes.
Under the worst conditions, the furnace must be shut down and the boiler tubes cooled to allow them to be manually shaken and scraped. The manual cleaning of the boiler tubes is a dangerous and time consuming operation. Further, the operation is extremely costly and causes a disruption in the production of power for the duration of shutdown.